[cairo] Text scaling and hinting

[Robert O'Callahan]

I think I can work around the current Cairo API. String drawing would
fix all the glyph positions based on a cairo_scaled_font_t chosen to be
the definitive metrics for the font, and then call cairo_show_glyphs.
But I'd certainly prefer to delegate the problem to Cairo, since this
solution might be suboptimal in output quality and performance.

A related question: I'm guessing that cairo_set_font_size is just
shorthand for cairo_set_font_matrix (i.e., it's not cumulative with
cairo_set_font_matrix, and only a font matrix is stored internally). Am
I right? It's not clear from cairo.h.

Rob